The present invention relates to a technique effective if applied to a semiconductor integrated circuit with a PLL (Phase Locked Loop) built therein, and relates to, for example, a technique effective if used in a semiconductor integrated circuit for wireless communication, which incorporates therein a PLL circuit that generates an oscillation signal having a predetermined frequency, which is combined with a receive signal or a transmit signal for wireless communication.
In a wireless communication system like a cellular phone, a high-frequency semiconductor integrated circuit (hereinafter called “high-frequency IC”) has been used which includes a PLL circuit including an oscillator that generates a local oscillation signal having a predetermined frequency, combined with a receive signal or a transmit signal and which modulate the transmit signal and demodulates the receive signal. As the cellular phone, there has heretofore been known a dual band type cellular phone capable of treating signals lying in two frequency bands like, for example, GSM (Global System for Mobile Communication) of a 880 to 915 MHz band and DCS (Digital Cellular System) of a 1710 to 1785 MHz band. In such a dual band type cellular phone, there is known one wherein one PLL circuit is capable of adapting to two bands by switching the frequency of the PLL circuit.
In recent years, however, there has been a demand for a triple band type cellular phone capable of treating signals employed in a PCS (Personal Communication System) of a 1850 to 1915 MHz band in addition to GSM and DCS. It is expected that the cellular phone needs to be capable of adapting to many types further in future years. A voltage-controlled oscillator (VCO) used in the cellular phone adaptable to such plural types needs to have a wide oscillation frequency range. When one VCO intends to adapt to all frequencies now, the sensitivity (hereinafter called “control sensitivity”) of an oscillation frequency to a control voltage of the VCO becomes high, thereby causing trouble that it is susceptible to external noise and a variation in power supply voltage.
Thus, there has been proposed the invention wherein the control sensitivity of the VCO can be reduced while holding a desired oscillation frequency range by allowing the VCO to be used by being switched to a plurality of frequency bands (e.g., 16 frequency bands) (refer to a patent document 1 (Japanese Unexamined Patent Publication No. 2003-152535)). Incidentally, the invention of the previous application adopts a type wherein actual frequencies are measured over all frequency bands for the VCO in advance before the start of operation and stored in a memory, and when oscillation frequency information is given, the oscillation frequency information and the frequency measured value in the memory are compared to decide the optimum frequency band.
In the cellular phone, however, efforts have recently been made to incorporate many circuits into one or several semiconductor integrated circuits as much as possible in order to reduce the number of parts and bring the device into less size and cost. As one of them, there is known an attempt to build a loop filter provided on a PLL loop lying in a high-frequency IC into a semiconductor chip.